The present invention relates generally to exposure apparatuses, and more particularly to an exposure apparatus that is used to expose objects, such as a single crystal plate for a semiconductor wafer, a glass plate for a liquid crystal display (“LCD”). The present invention is suitable, for example, for an exposure apparatus that uses light having a wavelength of 200 nm or smaller as an exposure light source.
A projection exposure apparatus that transfers a circuit pattern on a reticle or a mask onto a wafer and the like has conventionally been used to fabricate fine semiconductor devices, such as a semiconductor memory and a logic circuit, with photolithography technology.
The resolution (or critical dimension to be transferred) of the projection exposure apparatus is proportional to a wavelength of the exposure light and inversely proportional to its numerical aperture (“NA”). Therefore, the shorter the wavelength is, the better the resolution is. Recent demands for fine processing to the semiconductor devices have promoted use of shorter wavelengths of the exposure light. In recent years, an exposure light source has been in transition from the conventional ultrahigh pressure mercury lamp (g-line (with a wavelength of approximately 436 nm), i-line (with a wavelength of approximately 365 nm)) to KrF excimer laser (with a wavelength of approximately 248 nm) and ArF excimer laser (with a wavelength of approximately 193 nm), which have shorter wavelengths. F2 laser (with a wavelength of approximately 157 nm) has been being reduced to practice.
The exposure light with a wavelength under 200 nm has an emission spectrum that overlaps an absorption band of oxygen, is subject to absorptions by oxygen, and results in reduced exposure light quantity or transmittance on the object and lowered throughput of the apparatus. In addition, ozone created from oxygen that has absorbed the exposure light absorbs more exposure light, and various ozone-induced products adhere onto optical elements' surfaces, lowering the optical performance of an optical system. In particular, a spectrum band of F2 laser also overlaps an absorption band of water. Therefore, water as well as oxygen absorb the exposure light, and lower its transmittance, the throughput of an apparatus, and the optical performance of an optical system.
A projection optical apparatus that uses as a light source ArF laser or F2 laser or the like thus needs to purge oxygen and water from an exposure light's optical path by supplying photochemically non-reactive, inert gas (such as nitrogen gas, helium gas and argon gas) to an atmosphere of the optical path of an illumination optical system and a projection optical system. For example, an exposure apparatus has been proposed which maintains a projection optical system in an enclosed space from its side end close to a photosensitive substrate to its end close to the photosensitive substrate, and purges the enclosed space and the projection and illumination optical systems with inert gas (see, for example, Japanese Patent Application Publication No. 8-279458).
Deposits caused by impurities included in the surrounding atmosphere, such as ammonium sulfide and silicon oxide, adhere onto the surfaces of optical elements in an exposure apparatus. These deposits contaminate the optical elements, and deteriorate the optical performance, such as the lowered or uneven light intensity. One impurity source is gas emissions from resist on a photosensitive substrate. In particular, an optical element in the projection optical system located closest to the photosensitive substrate always suffers from gas from the resist, and remarkably lowers its optical performance.
There has been proposed an exposure apparatus that detachably arranges a parallel plate as an optical element in an optical path between a projection optical system and a photosensitive substrate, and replaces the optical element with a new one when it is contaminated (see, for example, Japanese Patent Application Publication No. 4-365050).
There has also been proposed an exposure apparatus that prevents oxygen and water existing, etc. in an optical path from absorbing the exposure light, and deposits from contaminating an optical element (see, for example, Japanese Patent Application Publication No. 2001-118783). This exposure apparatus will be explained with reference to FIG. 11, which is a schematic structure of a part of a conventional exposure apparatus 1000.
Referring to FIG. 11, a projection optical system 1100 projects a pattern image of the reticle (not shown) under exposure light EL in a vacuum UV region onto a wafer 1200 on a wafer-stage 1250 in a wafer chamber 1300. A blower plate 1400 has an opening 1410 that opens an optical path of the exposure light EL, and is placed between the projection optical system 1100 and the wafer 1200. The blower plate 1400 supplies inert gas from an inlet 1430 to a first space H1 in one direction and effectively exhausts gas from the wafer 1200, after exhausting the atmosphere in the first space H1 from an outlet 1420. The blower plate 1400 also supplies inert gas into a second space H2 at a lower position.
Disadvantageously, the exposure apparatus in Japanese Patent Application Publication No. 8-279458 has a thin air layer between the photosensitive substrate and the neighborhood of the photosensitive substrate and cannot completely purge with inert gas the space between them, causing the exposure light to be absorbed by oxygen, the water or the like. Especially, F2 laser as exposure light is greatly absorbed even in a thin air layer, and its decreased light quantity reaching the object greatly lowers the throughput of the apparatus.
On the other hand, the exposure apparatus in Japanese Patent Application Publication No. 4-365050 cannot prevent contaminations of the parallel plate as an optical element, disadvantageously causing increased running cost due to frequent exchanges of the optical element. Especially, a glass material applicable to a wavelength of F2 laser or the like is limited to calcium fluoride (CaF2), and increases the processing cost as well as the material cost due to higher surface precision of the optical element for the higher resolution. Therefore, the exposure apparatus is not viable unless the parallel plate as an optical element is free of contaminations.
The exposure apparatus published in Japanese Patent Application Publication No. 2001-118783 arranges the inlet 1430 and the outlet 1420 in the first space H1 (between the projection optical system 1100 and the blower plate 1400), and flows gas in one direction. However, the outlet 1420 located in the first space H1 easily takes oxygen or water outside into the first space H1, and disadvantageously lowers the purge efficiency. A proposed configuration that eliminates the blower plate 1400 and the outlet 1420 from a downstream of an exposure area is not also viable, because it takes surrounding gas and remarkably lowers the gas blowing efficiency at the opening 1410.